Reliability engineering attempts, at a minimum, to quantify failure rates of a particular type of component or system and to identify the failure mechanism involved. Since most failure mechanisms are not a priori known, reliability engineering must at a minimum experimentally measure the lifetime to failure of the part. Thereafter, it is desirable to identify why it failed and attempt to eliminate the failure mechanism from the equipment.
The local-exchange telephone industry imposes aggressive lifetime requirements on most of its hardware, on the order of several decades. It is obviously of little use to insert the part into a normal working environment and wait for such periods of time for a failure to occur in order to first identify the failure rate or mechanism. Accordingly, electronic components and even systems are often stressed at higher temperatures than ambient or operational temperature with the hope of accelerating a weakness or flaw that will cause the component or system to fail before it is shipped to the customer. This type of accelerated testing will only be successful if the failure mode is accelerated by increasing the temperature, which is not unusual since most failure modes involve a thermally activated process. If the part is simply left in an inoperative state at the elevated temperature and thereafter tested, the test is called a static or storage accelerated test.
On the other hand, if a part is used in an electrical circuit, a more realistic test activates or otherwise electrically stresses the part with a voltage while the part remains at an elevated temperature. This combined test is called a temperature-bias (TB) test.
If the parts may be used in a severe environment, such as high humidity, then the high temperature, electrical bias, and humidity are combined in one stressing oven or chamber to further accelerate failure. Such temperature-humidity-bias (THB) testing has shown that the combination of heat and humidity degrade components that are not sealed in an environmentally secure, i.e., hermetic package that does not allow the ambient atmosphere to penetrate to the active or most sensitive portion of the device being tested.
Often, a manufacturer will package electronic components in non-hermetic packages, such as a plastic epoxy packages, for a variety of reasons ranging from cost saving to improved high-frequency performance. A controversy remains about the validity of correlating THB testing to the typical environment that a component experiences. A number of experimenters have tried to extrapolate THB data to TB data or to ambient conditions. The correlation is important because, if correctly done, it would allow the manufacturer or customer of a part that is specified for a lifetime of, say, 25 years to know the performance of the part over that extended period.
Investigators have created even more harsh environments by adding yet another variable to THB testing, specifically, elevated pressure. Elevated pressure tests are particularly applicable to non-hermetic packages. Components are inserted into a pressure vessel or pressure cooker where they experience temperature, humidity, pressure, and electrical bias. At higher than normal atmospheric pressure, moisture may diffuse into a component and cause failure mechanisms similar to those found under THB, but at a faster rate. The equipment used to implement temperature-humidity-pressure-bias (THPB) testing is often called a highly accelerated stress tester (HAST).
The military has imposed a thermal shock requirement upon electronic equipment, as specified in paragraph 3.1 of MIL-STD-883C, Method 1011.9. The test standard does not require real-time testing of the device or even biasing during the temperature cycling. Typical prior-art thermal-shock chambers used to apply the standard mechanically move samples between hot and cold chambers, for example, by an elevator. Not only are such apparatus bulky, further their mechanical movement complicates the cabling required for the electrical and optical signals to and from device under in situ testing.